The class of “conjugated linoleic acids” (CLA=conjugated linoleic acid) comprises numerous isomers of C18:2 fatty acids of which the double bonds form a conjugated p-system. An analytical separation of the isomers was recently described in ANALYTICA CHIMICA ACTA: Chromatographic separation and identification of conjugated linoleic acid isomers: Roach, J. A. G., Mossoba, M. M., Yurawecz, M. P., Kramer, J. K. G.; 465 (1–2): 207–226, Aug. 16, 2002. Whereas natural sources, such as dairy products, mainly contain the c9,t11-isomer (pansenoic acid), commercial syntheses normally yield a 1:1 mixture of c9,t11- and t10,c12-isomer. Both isomers show specific physiological activities, for example preference for muscle development over the replacement of fatty tissue, strengthening of the immune system, positive effects on bone development and anticarcinogenic activity. The described properties make a mixture of these two isomers particularly interesting for use as food additives. However, a basic requirement for use in this field is high isomer purity, i.e. the absence of unwanted CLA isomers—especially the c11,t13-CLA and trans, trans-CLAs. These are formed at temperatures above 130° C. by isomerization of c9,t11- and t10,c12-CLA.
All the industrial processes presently used for the production of CLA provide them in the form of the free acid or its esters. However, fatty acids are naturally taken up mainly as fats and oils and, even in food supplementation, the triglyceride is the preferred CLA derivative. Besides far better sensory properties, its very considerable structural and physical similarity to conventional triglycerides—especially oils rich in linoleic acid, such as thistle oil and sunflower oil—provides for ready incorporation in a number of fat-containing preparations. In addition, triglycerides are far more stable to oxidation than the corresponding free fatty acids.
Since there is no known process for isomerizing linoleic acid groups of an oil with the triglyceride structure intact, CLA triglycerides are currently produced by transesterification of CLA/CLA esters and glycerol. For the reasons described above, this requires relatively mild conditions under which only enzymatic catalysts presently show adequate activity. However, serious disadvantages of these enzyme-catalyzed reactions are the long reaction time of several days, very high enzyme costs and the difficulties involved in handling the enzyme. The lipase-catalyzed inter-esterification of long-chain fatty acids or alkyl esters thereof with triglycerides of short-chain fatty acids is known, for example, from WO 90/12858.
Many processes for the production of acetoglycerides, more particularly stearyl acetyl glycerides, are based on the transesterification of corresponding fats with triacetin (triacetyl glyceride). The catalysts used are mainly metal soaps. In some cases, the reactions are carried out at 200 to 260° C., as described in U.S. Pat. No. 6,124,486. At lower temperatures, the poor miscibility of the triacetin with triglycerides carrying long-chain fatty acids (C16–C22) leads to significant losses of reactivity and hence yield. This limitation can be avoided by the addition of C3–C10 triglycerides. International patent application WO 94/18290 and U.S. Pat. No. 5,434,278 disclose the inter-esterification of triacetin and triglycerides with long-chain C16–C22 fatty acid residues to form mixed triglycerides with long-chain and short-chain fatty acid residues, characterized in that triglycerides containing saturated C3–C10 fatty acid residues are added to the reaction. In this way, the reaction takes place rapidly in a single phase, without solvents and without intensive mixing.
Accordingly, the problem addressed by the present invention was to provide a process for the production of CLA triglycerides which would be distinguished by high profitability as reflected in short reaction times, high yields and inexpensive starting materials. The CLA triglycerides formed would have high isomer purity, i.e. a low content of trans, trans isomers and c11, t13 isomers.